Section E – Spectrometric techniques
E10 INFRARED AND RAMAN
SPECTROMETRY: PRINCIPLES
AND INSTRUMENTATION
Principles The vibrational levels of molecules are separated by energies in the infrared (IR)
region of the electromagnetic spectrum. That is, in the wavenumber range
from 13 000 to 10 cm-^1 , or between 0.8 and 1000 mm on the wavelength scale.
For convenience, this large region is divided into near IR, or NIR(13 000-
4000 cm-^1 ), mid IR(4000-400 cm-^1 ) and far IR(400-10 cm-^1 ).
Molecules contain bonds of specific spatial orientation and energy. These
bonds are seldom completely rigid, and when energy is supplied, they may
bend, distort or stretch. A very approximate model compares the vibration to
that of a harmonic oscillator, such as an ideal spring. If the spring has a force
constant, k, and masses mAand mBat the ends, then the theoretical vibration
frequency nis given by:
n=(1/2p)√(k/m)
where m=mA.mB/(mA+mB) is called the reduced mass.
Each type of molecular vibration is characterized by a vibrational quantum
number, v. For a simple stretching vibration, there is a series of levels whose
energy is given approximately by
E =hn 0 .(v+^1 ⁄ 2 )
This means there is a set of levels spaced in energy by hn 0 or in wavenumber by
n
_
0. The selection rule for an ideal harmonic oscillator allows transitions where
Dv=±1, giving a single, fundamental vibrational absorption peak.Key Notes
Vibrational transitions in molecules cause absorption in the infrared
region of the electromagnetic spectrum. They may also be studied using
the technique of Raman spectrometry, where they scatter exciting
radiation with an accompanying shift in its wavelength.Vibrational spectra give information about the functional groups in
molecules, and the observed group frequencies are affected by molecular
interactions such as hydrogen bonding.Infrared and Raman instruments include a radiation source, a means of
analyzing the radiation and a detection and data processing system.
Additionally, sampling methods to deal with gases, liquids, solids,
microsamples and mixtures are available.E11 Infrared and Raman spectrometry: applications
applications (E11) spectrometry (F4)PrinciplesGroup frequenciesInstrumentation